Nickel Hollow Spheres Concatenated by Nitrogen-Doped Carbon Fibers for Enhancing Electrochemical Kinetics of Sodium-Sulfur Batteries

The high energy density of room temperature (RT) sodium-sulfur batteries (Na-S) usually rely on the efficient conversion of polysulfide to sodium sulfide during discharging and sulfur recovery during charging, which is the rate-determining step in the electrochemical reaction process of Na-S batteries. In this work, a 3D network (Ni-NCFs) host composed by nitrogen-doped carbon fibers (NCFs) and Ni hollow spheres is synthesized by electrospinning. In this novel design, each Ni hollow unit not only can buffer the volume fluctuation of S during cycling, but also can improve the conductivity of the cathode along the carbon fibers. Meanwhile, the result reveals that a small amount of Ni is polarized during the sulfur-loading process forming a polar Ni-S bond. Furthermore, combining with the nitrogen-doped carbon fibers, the Ni-NCFs composite can effectively adsorb soluble polysulfide intermediate, which further facilitates the catalysis of the Ni unit for the redox of sodium polysulfide. In addition, the in situ Raman is employed to supervise the variation of polysulfide during the charging and discharging process. As expected, the freestanding S@Ni-NCFs cathode exhibits outstanding rate capability and excellent cycle performance.